Micrel, Inc.
MIC2808
Inductor Selection
Component Selection
The MIC2808 is designed for use with a 4.7µH inductor.
Proper selection should ensure that the inductor can
handle the maximum average and peak currents
required by the load. Maximum current ratings of the
inductor are generally given in two methods; permissible
DC current and saturation current. Permissible DC
current can be rated either for a 40°C temperature rise
or a 10% to 20% loss in inductance. Ensure that the
inductor selected can handle the maximum operating
current. When saturation current is specified, make sure
that there is enough margin, so that the peak current will
not saturate the inductor. Peak inductor current can be
calculated as follows:
Output Capacitor
LDO1 output requires a 2.2µF ceramic capacitor, while
the LDO2 and DC/DC regulator outputs require a 1µF
ceramic capacitor. All output capacitor values can be
increased to improve transient response, but perform-
ance has been optimized for a 2.2µF ceramic capacitor
for LDO1 and 1µF ceramic capacitors for both the
DC/DC regulator and LDO2. X7R/X5R dielectric-type
ceramic capacitors are recommended because of their
temperature performance. X5R/X7R-type capacitors
change capacitance by 15% over their operating
temperature range and are the most stable type of
ceramic capacitors. Z5U and Y5V dielectric capacitors
change value by as much as 50% to 60% respectively
over their operating temperature ranges.
⎛
⎞
VOUT
⎜
⎟
⎟
VOUT 1−
⎜
V
IN
⎝
⎠
IPK = IOUT
+
2× f ×L
Input Capacitor
IPK:
Peak Inductor Current
Output/Load Current
Input Voltage
For optimal bypassing a minimum 1µF ceramic, 2.2µF
recommended, should be placed as close as possible to
the VIN pin. X5R or X7R dielectrics are recommended
for the input capacitor. Y5V dielectrics lose most of their
capacitance over temperature and are therefore, not
recommended. For high frequency filtering a minimum
1µF is recommended close to the VIN and PGND pins.
Smaller case size capacitors are recommended due to
their lower ESR and ESL. Please refer to the PCB layout
section for an example of an appropriate circuit layout.
IOUT
VIN:
:
VOUT
f:
:
Output Voltage
Switching Frequency of PWM Regulator
Inductor Value
L:
M9999-071709-A
July 2009
14
MICREL [ MICREL SEMICONDUCTOR ]
